1. Field of the Invention
This invention relates to active acoustic attenuation systems which reduce noise by generating canceling sound substantially equal in magnitude but opposite in phase to the noise, and in particular to such systems for reducing tonal noise produced by small cooling fans for electronic equipment such as computers
2. Background Information
Many computers and other electronic equipment use forced ventilation as a means of cooling the electronic components. The primary complaint from computer operators is that the ventilation fans produce annoying tones. One solution places the operators in a separate room from the computer. However, with the advent of the personal computer and the workstation, the operator must be near the device
Fan noise originates from two sources. The first source is the turbulent flow of air as it is exhausted by the fan. This noise is random and can be reduced by proper fan and grill design. The second source of noise is called blade pass noise. As the fan blade passes a nearby support, a pressure wave is produced. Since the fan rotates at a constant speed, a periodic sequence of pressure waves produces a fundamental tone plus higher order harmonics.
Common solutions to fan noise control involve the use of sound absorption devices and repositioning of the fans. Intake and exhaust silencers reduce the noise produced by fans; however, these solutions sacrifice airflow, i.e., reduce cooling, for noise reduction.
Active control of noise in enclosed systems such as ducts has been known for sometime. In this type of noise control, a destructive interference pattern is generated by a speaker positioned near the noise source, which radiates a signal that is 180 degrees out of phase with the noise. The signal from the speaker cancels the noise. Often, the source noise is sampled by an input microphone to generate an input reference and the resultant sound produced by the combination of the canceling wave from the speaker with the noise is sampled by an error microphone. The difference between the two sampled sounds is used to generate the signal driving the speaker. Typically, the control system which generates the speaker signal utilizes an adaptive filter which accommodates for the time required for sound to propagate from the source and from the speaker to the error microphone. A common type of adaptive filter used in this application is the LMS (least means square) adaptive filter. An example of such a filter is described in U.S. Pat. No. 4,473,906. A modification of this filter is the filtered-X LMS adaptive filter which overcomes difficulties in obtaining convergence of the adaptive filter to control tonal noise in the presence of broad band noise. Such a filtered-X LMS adaptive filter is described by Bernard Widrow and Samuel D Stearns in Adaptive Signal Processing, Prentice-Hall, Inc., 1985, pp 288-292.
Active control systems which use the source noise as an input reference signal must contend with feedback from the speaker to the input microphone. Where fan blade pass noise is to be attenuated, this problem can be avoided by using a signal representative of the rotational speed of the fan as the input reference signal for the control system, as noted in U.S. Pat. No. 4,677,677. An experimental system which uses a toothed wheel and an optical detector to generate a square wave signal from which the blade passing frequency can be derived is described by G. H. Koopmann and D. J. Fox, in Active Source Cancellation of the Blade Tone Fundamental and Harmonics in Centrifugal Fans Journal of Sound and Vibration, Academic Press Limited, 1988, pp 209-220. That system, however, uses two 2-channel phase shifting phase locked loops to generate signals for two speakers used to attenuate the fundamental frequency and only one selected harmonic of the blade pass noise in a centrifugal fan. Microphones are used only to monitor the resultant sound and not to control the phase locked loops which must be manually adjusted.
There is a need for an improved active acoustic attenuation system for attenuating tonal noise generated by rotating equipment.
There is a further need for such a system which attenuates the fundamental and all harmonics of tonal noise generated by the fans.
There also is a need for such a system which can be implemented economically.